Ceramic tubes have found a use in the manufacture of Solid Oxide Fuel Cells (SOFCs). There are several types of fuel cells, each offering a different mechanism of converting fuel and air to produce electricity without combustion. In SOFCs, the barrier layer (the “electrolyte”) between the fuel and the air is a ceramic layer, which allows oxygen atoms to migrate through the layer to complete a chemical reaction. Because ceramic is a poor conductor of oxygen atoms at room temperature, the fuel cell is operated at 700° C. to 1000° C., and the ceramic layer is made as thin as possible.
Early SOFCs were produced by the Westinghouse Corporation using long, fairly large diameter, extruded tubes of zirconia ceramic. Typical tube lengths were several feet long, with tube diameters ranging from ¼ inch to ½ inch. A complete structure for a fuel cell typically contained roughly ten tubes. Over time, researchers and industry groups settled on a formula for the zirconia ceramic which contains 3 mol % Y2O3. This material is made by, among others, Tosoh of Japan as product TZ-3Y.
Another method of making SOFCs makes use of flat plates of zirconia, stacked together with other anodes and cathodes, to achieve the fuel cell structure. Compared to the tall, narrow devices envisioned by Westinghouse, these flat plate structures can be cube shaped, 6 to 8 inches on an edge, with a clamping mechanism to hold the entire stack together.
A still newer method envisions using larger quantities of small diameter tubes having very thin walls. The use of thin walled ceramic is important in SOFCs because the transfer rate of oxygen ions is limited by distance and temperature; if a thinner layer of zirconia is used then the final device can be operated at a lower temperature while maintaining the same efficiency. Literature describes the need to make ceramic tubes at 150 μm or less wall thickness. These new thin-wall tubes will be seen to be the subject of the present invention.
Extrusion is the most common method for making ceramic tubes. In this approach, ceramic particles are mixed with an organic binder, often a waxy material, and the material is pressed through a circular opening. The problems with this method include (i) maintaining straightness during the firing process, (ii) obtaining thin walls with no defects, and (iii) preventing sagging of the circular cross-section into an oval shape.
Numerous patents describe methods of improving the manufacture of extruded thin-wall ceramic tubes. Continuous firing in an attempt to create long tubes has been described in U.S. Pat. No. 5,227,105. Sands, et al. describe in U.S. Pat. No. 4,395,231 the rotation of a tubular furnace as the tubular devices are passed through, whereby the speed going into the sintering furnace is faster than the speed coming out of the furnace so as to account for the shrinkage of the ceramic. In U.S. Pat. No. 4,770,631, Hell et al. describe a method of hanging tubes vertically during sintering. In U.S. Pat. No. 5,935,513, Martreuil et al. describe firing a ceramic tube inside of a larger ceramic support tube. Other patents, including U.S. Pat. No. 4,579,707 to Kobayashi et al., describe methods of improving the stiffness of the un-fired tube by using a thermosetting organic binder, and then applying heat immediately after extrusion.
In efforts to make small, thin walled tubes, the extrusion process faces several challenges. One is that the tubes can warp or twist during binder removal. This problem may be due to the fact that the binders commonly used for extrusion do not maintain their strength throughout the binder removal process before sintering. Another problem relates to the production of the thin walls themselves. At a thickness of 150 μm or less, a fairly small defect, such as an air bubble or a binder inclusion, can cause a defect in the final tube, creating a leak that would be considered catastrophic in a SOFC. Another practical problem with extruding thin walled tubes is that they are mechanically weaker than a thicker tube, which makes mounting difficult.
Henrik Raeder of the Center for Industrial Research in Norway has described the use of tape cast ceramics for making thin walled tubes. Tape casting involves evenly coating a horizontal surface with a ceramic slurry, drying, then removing the dried film. The slurry is prepared by dispersing ceramic in an organic binder, often a mixture of polyvinyl butyryl in solvent. Raeder described using 8 to 20 mm wide strips of tape, and winding them around alumina or glass rods. The wrapped material had an overlap of 1 to 3 mm. The diameter of the rods was 2 to 6 mm. After forming the tubes, they were slipped off the ends of the rods. Except in the areas of the seam, these tube walls were one thickness of cast ceramic, and they had trouble maintaining perfect circular form.
Two methods were used by Raeder to seal the tube along the wrapped seam. One was based on applying ethanol to the seam, which dissolved the binder and made it stick to the next layer. Another method was to apply thinned slurry to the seam, which had the advantage of both sealing the seam and coating it with additional ceramic.